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Related Parties - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended
Oct. 26, 2015
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Related Party Transaction [Line Items]        
Fees receivable   $ 503,234   $ 501,397
Investment Banking and Other Advisory Fees [Member]        
Related Party Transaction [Line Items]        
Revenue   335,804 $ 408,726  
Investment Banking and Other Advisory Fees [Member] | Chairman and CEO [Member]        
Related Party Transaction [Line Items]        
Revenue   3,000    
Sponsored Funds [Member]        
Related Party Transaction [Line Items]        
Revenue   $ 144,170 $ 181,309  
Type of Revenue [Extensible List]   us-gaap:AssetManagement1Member us-gaap:AssetManagement1Member  
Fees receivable   $ 52,040   59,304
LTBP Trust [Member]        
Related Party Transaction [Line Items]        
Amount of cash savings in income or franchise tax that would be realized as a result of increases in tax basis and certain other tax benefits related to the amended and restated tax receivable agreement 45.00%      
Cash tax saving that may arise from tax benefits attributable to payments under the amended and restated tax receivable agreement 85.00%      
Cumulative liability relating to obligations under Amended and Restated Tax Receivable Agreement   246,966   $ 270,640
Payments under tax receivable agreement   $ 23,674